Chemical coagulants have long been used in water and waste water treating to induce flocculation of particles suspended in the raw water to be treated. This aggregation of suspended particles allows for more efficient sedimentation and/or filtering downstream. For best results, the initial mixing of the chemical coagulant with the raw water should occur as rapidly as possible to form a homogenized mixture within one or two seconds.
The principal objective of this rapid or flash mixing is to ensure a homogeneous coagulation by completely uniform dispersion of the coagulant throughout the water. In this way, the coagulant can make contact with the maximum number of suspended particles prior to the completion of hydrolysis, enabling intermediate complexes to destabilize the suspended particles initiating aggregation. This chemistry of destabilization sets some of the requirements for efficient rapid mixing.
Chemical coagulants should be dispersed in an unblended stream of raw water. Dispersing chemicals into a blended or partially blended stream (backmixing) can lead to poor destablization of a fraction of the particles because some might have insufficient surface coverage while others might have too extensive surface coverage by adsorbed chemical species. This wastes chemicals and results in less effective floc formation for a given amount of a coagulant.
Stagnation time, defined as the amount of time that elapses from the addition of coagulant to the start of mixing, should be reduced for most effective coagulation.
From a mechanical point of view, a rapid mixing device should be simple, practical and relatively inexpensive and should not create appreciable head losses.
Through the years, in attempting to meet these chemical and mechanical requirements, many devices have been employed to provide the rapid mixing needed for chemical dispersion. These include the weir, the Parshall Flume, rapid mixing chambers equipped with mechanical rotary mixing devices such as propellers or turbines and in-line blenders. More recently, hydraulic diffusion flash mixing has been used as a method providing rapid mixing without appreciable head losses and lower operating and maintenance costs than mechanical methods. This method also provides more efficient rapid mixing with reductions of 20 to 30 percent in chemical coagulant consumption over mechanical methods.
Generally hydraulic diffusion flash mixing operates by drawing off a portion of the raw water to be treated into a carrying water loop. The chemical coagulant to be dispersed is added to this drawn off portion. The mixture of raw water and coagulant is then injected into the remainder of the raw water through a diffuser. A pump in the carrying water loop provides the pressure for injection.
Usually the diffuser is a radial jet diffuser which injects the raw water and coagulant mixture perpendicular to the flow direction of the remaining raw water from several nozzles equally spaced about the circumference of a tube placed in the center of the pipe carrying the remaining raw water. Radial injection can also occur by injection perpendicular to the flow direction from nozzles equally spaced about the pipe periphery. This alternate reduces head losses but because turbulent velocity intensity increases from the center of the pipe toward the wall, a jet introduced at the center of the pipe receives more mixing than one being introduced from the wall, so central injection is preferred.
Sometimes the diffuser is a conical jet diffuser which injects the raw water and coagulant mixture parallel to the flow direction of the remaining raw water through a single nozzle, directed either upstream or downstream with the flow, located in the center of the pipe carrying the remaining raw water. These alternatives are not preferred because a nozzle directed upstream causes backmixing and one directed downstream requires a long time for complete mixing. Both these situations do not provide most efficient coagulant use.
Problems have developed with hydraulic flash diffusion mixing in some applications. Where hardness exists in the raw water to be treated, addition of coagulant in the carrying water loop has led to clogging of the diffuser nozzles. This clogging requires periodic plant shutdowns be scheduled to clean the diffuser resulting in greatly increased operating and maintenance costs. In one case, this cleaning was necessary so frequently (once a month) the system had to be abandoned for less efficient mechanical methods.